Telephone instrument circuit



Nov. 8, 1938. G. w. SUTTON El AL 2,136,345

TELEPHONE INSTRUMENT CIRCU IT Filed June 22, 1936 kind in which a singlemoving electrode is situ- Paten'ted Nov. 8, 1938 TELEPHONE INSTRUMENTCIRCUIT I George William Sutton, London, and William Herbert Grinsted,Bromley, England, assignors to Siemens Brothers & Company Limited, London, England Application June 22, 1936, Serial No. 86,568 In GreatBritain July 1, 1935 series together form the arms of a four-armedbridge, a circuit for feeding direct current to the microphone beingconnected to the two junction points where a condenser is connected toone of the said halves or elements and an output circuit (e. g. theprimary winding of a transformer), being connected to the other pair ofjunction points.

In another arrangement which may be employed in practice, the two halvesor microphone elements of the microphone are connected in series acrossa battery, and the junction between the two elements is connected via anoutput circuit (e. g. the primary winding of a transformer) and acondenser connected inseries to the centre point of the battery.

The features of the invention are exemplified in the specificembodiments of it which will now be described with reference to theaccompanying drawing. Fig. 1 of the drawing shows a subscribersinstrument circuit arranged for central battery working and Fig. 2 showsa subscribers instrument circuit arranged for local battery working. Thefigures are schematic in character, switch-hook contacts and the likebeing omitted.

In the circuit shown in Fig. 1, two similar condensers CA and CBconnected in series and the two halves or microphone elements of thedifferential variable-resistance microphone M con- 4 Claims.

(The present invention relates to telephone instrument circuits, andmore particularly to circuit arrangements for use withVariable-resistance transmitters or microphones of the difierentialtype.

By a variable-resistance microphone of the diflerential type is meantone which comprises what are in eifect two microphone elements, theseelements being arranged so that during the operation of the microphonethe resistance of one element is increased as the resistance of theother is decreased, and vice versa. Such microphones are usually (butnot necessarily) of the ated between two fixed electrodes, the twomicrophone elements being formed by the variableresistance path betweenthe moving electrode and one fixed electrode and the variable-resistancepath between the moving electrode and the other fixed electrode.

According to the invention, the circuit of a variable-resistancemicrophone of the difierential type is such that direct current whichpasses for feeding purposes through either half or microphone element ofthe microphone must also pass through the other half or microphoneelement irrespective of whether or not the resistances of the two halvesor elements are equal in value. The provision of this feature helps tomaintain the two halves of the transmission system. in balance.

In carrying out the invention, we arrange that the two halves ormicrophone elements of the microphone are connected in series acrossleads connected directly or indirectly to a battery or other source ofmicrophone feeding current, and also arrange that the passage of directcurrent through an output circuit (e. g. the primary winding of atransformer) connected to the junction between the two elements (e. g.connected to the moving electrode of a microphone comprising two fixedelectrodes and an intermediate moving electrode) is blocked by one ormore condensers. The said condenser or condensers may be arranged toplay an important part in determining the response-frequencycharacteristic of the microphone system. For example, it may be arrangedthat a high-pass filter effect is obtained, so that the microphonesystem provides good speech articulation but does not transmitlow-frequency room noise efficiently.

In one arrangement which may be employed in practice, two condensers ofequal capacity connected in series and the two halves or microphoneelements of the microphone connected in armed bridge. The microphone maybe constructed in one of the manners described in the application ofWilliam E. Goodwin, Serial No. 62,121, filed February 3, 1936. Lineterminal Ll is connected via a choke coil CH to one juncconnected to theopposite junction point via transformer TR, the two junction pointsreferred to being those where a condenser is connected to a microphoneelement. A condenser CC and the receiver R are connected in seriesbetween line terminalLl and the said opposite junction point, thereceiver being shunted by a circuit comprising an impedance in serieswith winding (III) of transformer TR. The impedance may as shown be acondenser CD in series with a non-inductive resistance S, in which casethe resistance may be formed by a noninductive spool carried by or woundon transformer 'I'R. The pair of junction points of the bridge whichhave not yet been referred to are connected to winding (1) of thetransformer. Each of the condensers CA, CB, and CC may have a capacityof two microfarads and the condenser CD may have a capacity of nected inseries together form the arms of a fourtion point of the bridge and lineterminal L2 is winding (II) of a three-winding anti-sidetone half amicrofarad. A magneto bell B is connected between terminal L2 and thejunction of condenser CC and receiver R.

When the instrument circuit is in the conversational condition, directcurrent is fed to the microphone from the exchange battery in a circuitwhich includes terminal Ll, choke coil CH, the two halves or microphoneelements of the microphone M in series, winding (II) of transformer TR,and terminal L2. When the microphone is affected by speech, alternatingdifferences of potential are set up across winding (I) of thetransformer and the resulting speech currents flowing through thiswinding are transmitted to the line by the usual transformer action. Itwill be seen that the feed currents flowing through the two halves ormicrophone elements of the microphone must always be equal since anysteady direct current which passes through one must also pass throughthe other. It will further be seen that, since the impedances of thecondensers CA and CB must of necessity change with frequency and thesecondensers are included in the circuit of winding (I) of thetransformer, the said condensers may be arranged to play an importantpart in determining the response-frequency characteristic of themicrophone system. With a three-winding anti-sidetone transformer ofwell-known type and with condensers of the value quoted we have foundthat the effect of the condensers is to cause a progressive weakening ofthe response as the frequency is reduced from about 500 cycles persecond to a lower frequency.

In the circuit shown in Fig. 2, two similar condensers CA and CB and thetwo halves or microphone elements of the differentialvariable-resistance microphone M form a four-armed bridge which issimilar to that of the circuit of Fig. 1. In this case also themicrophone may be constructed in one of the manners described in theapplication previously referred to. Transformer TR. with its windings(I), (II), and (III) and condensers CC and CD, receiver R, bell B, andre sistance 3 correspond to the apparatus similarly designated in Fig.l. The two junction points of the four-armed bridge where a condenser isconnected to a microphone element are connected to the local battery LB.Each of the three condensers CA, CB, and CC may have a capacity of twomicrofarads and the condenser CD may have a capacity of half amicrofarad. It will be seen that in this circuit also the feed currentsflowing through the two halves or microphone elements of the microphonemust always be equal since any steady direct current which passesthrough one must also pass through the other, and the condensers may bearranged to play an important part in determining the response-frequencycharacteristic of the microphone system. The choke CH would not alwaysbe provided, its object when present being to provide a low-resistancepath for signal current in the case where the instrument circuit is usedon a central battery signalling C. B. S. system. The condenser CC isonly essential in the case of a C. B. S. system.

In a modification of the circuit just described, only one condenser isassociated with the microphone, the left-hand terminal of winding (I) oftransformer TR being connected via this condenser to the centre point ofthe battery, which must of course consist in this case of an even numberof cells. Here again any steady direct current which passes through onehalf or microphone element of the microphone must also pass through theother half or element.

We claim:

1. In a substation telephone circuit, a microphone of the differentialtype having two microphone elements, a transformer, one line conductorconnected to one of said microphone elements and the other lineconductor connected in series with a secondary winding of saidtransformer to the other microphone element, a connection comprising apair of condensers in series connected in bridge of said microphoneelements, one terminal of the primary winding of said transformerconnected to the midpoint between said two microphone elements, and theother terminal of said primary winding connected to the midpoint betweensaid two condensers.

2. A substation telephone circuit comprising a transformer, adifferential transmitter having two transmitter elements, a line, acircuit for said transmitter including one winding of said transformerand said two elements in series, said circuit connected to said line sothat the direct current flow thereover must pass through said elementsirrespective of the equalization of the resistance in said elements.

3. In a substation telephone circuit comprising a transformer, adifferential transmitter having two fixed electrodes and a movableelectrode, two condensers of equal capacity, a loop circuit includingsaid electrodes and said condensers in series, a direct current feedingcircuit for said transmitter, said feeding circuit connected to saidloop circuit so as to bridge said condensers, and an output circuitincluding one winding of said transformer connected to said loop circuitso as to bridge the midpoint between said condensers and said movableelectrode.

4. A substation common battery telephone circuit comprising atransformer, a differential microphone of the type having two fixedelectrodes and a single movable electrode, a line over which microphonecurrent is received, a circuit for said microphone including saidelectrodes and one winding of said transformer in series with said line,two condensers bridging said microphone electrodes, an output circuitincluding another winding of said transformer connected between saidcondensers and said movable electrode whereby the impedance of saidcondensers determine the response-frequency characteristics of saidtelephone circuit.

GEORGE WILLIAM SU'I'I'ON. WILLIAM HERBERT GRINSTED.

